In continuous time filter and gain amplifier applications, tunable Gm cells are often used. Environmental changes and process variations are accounted for by adjusting the transconductance (Gm) of an amplifier design. This process is called tuning. Tuned amplifiers are used in read/write channels in disk drives and for audio-frequency signal processing in communication circuits. In particular, tuning enables dynamic adjustment of circuit behavior to fulfill different functions within the same circuit. Designing Gm cells with a wide tuning range and low operating voltage compatibility is a challenge in circuit design.
FIGS. 10(a), (b), and (c) show configurations of Gm cells. In these devices, the Gm cell characteristics are set by one or more Gm setting devices and a tuning voltage. As shown in FIGS. 10(a) and 10(c), two Gm setting MOS devices are used. In circuit 1000, Gm setting devices 1030 and 1032 constitute a differential transistor pair in which each receive a portion of the tail current. Here, varying the tail current sets the transconductance of the Gm cell.
In circuit 1010, a sole Gm setting device 1034 is used. Here, varying a tuning voltage sets the transconductance of the Gm cell. In circuit 1020, two Gm setting devices 1036 and 1038 are incorporated and a tuning voltage sets the transconductance of the Gm cell. In these implementations, a wide tuning range corresponds to a wide voltage range for the tuning voltage or the tail current. For low voltage designs, this may be problematic due to the limited range of operable tuning voltages or tail currents.
Other Gm cells will apply different tuning voltages to different Gm setting devices. As shown in FIG. 11(a), circuit 1100 utilizes two pairs of Gm setting devices. Consequently, two tuning voltages are applied. Circuit 1100 allows a wider variety of Gm settings to be achieved in a smaller range of tuning voltages. This method, however, creates overlapping ranges of transconductance in the operation of the Gm setting devices, as shown in FIG. 11(b). This overlapping range creates added difficulty for circuit design.
Even with the improvements of circuit 1100, there remains a limited tuning range due to limited power supply voltage available. In order to obtain a wide transconductance range, the tuning voltage of the prior art tunable cells must have a wide range. This necessary range of tuning voltages creates poor distortion behavior because of a limited ratio between gate overdrive of the Gm setting MOS device and signal swing.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below relate to a switchable Gm cell. The Gm cell includes a plurality of Gm setting devices and a switching circuit operable to turn on and off the Gm setting devices. The switching circuit may further comprise a digital decoder. The preferred embodiments also relate to the design of Gm setting devices with differing channel widths and digital decoders operable to selectively turn on and off the Gm setting devices.
The preferred embodiments further relate to a method of driving a Gm cell utilizing a Gm setting code, decoding the Gm setting code, and turning on at least one of a plurality of Gm setting devices. Further, a tuning voltage may be altered to provide small adjustments to the overall transconductance of the Gm cell.
By incorporating a plurality of Gm setting devices, the operation of which is controlled by the use of a Gm setting code, the embodiments of the present invention enable a wide transconductance range, while allowing for a narrow tuning voltage range. In the preferred embodiments, the transconductance, Gm, of the circuit element may be applied in a linear, or stepped, manner without overlap between different voltage and/or Gm ranges. The monotone nature of the embodiments also allows for improved circuit design.
The features of the preferred embodiments are further described in the detailed description section below.